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He's polite, smart and a little nerdy. But as Rob Portman has climbed to the top of the list of potential running mates for Mitt Romney, the political world is now seeing a different side of him: the attack dog.

Just over the past week, the normally soft-spoken Portman has issued a series of blistering - for him anyway - attacks on Barack Obama.

By today's standards, these attacks are actually fairly tame (nothing compared to what Newt Gingrich has said about Romney, for example), but the tough new tone shows Portman is not afraid to play the traditional attack-dog role of a vice presidential candidate.

Here's a rundown of what we've heard from Portman of just the last week:

- In an interview with Bret Baier on Fox News, Portman turned a question about his lack of "sizzle" into a slam on Obama. "America made a decision in 2008 to go with a president who did have sizzle. And look, he was kind of a celebrity. He also had a very compelling message which was, remember this, 'I'm going to bring people together to solve problems.' Didn't happen. And it didn't happen because he didn't have the experience, he didn't have the record, he didn't have the policies to do it."

- A few days later, Portman accused the President of offering only "lofty rhetoric and poll-tested platitudes" when he visited Ohio. "Instead of changing course, President Obama wants to double down with more job-killing taxes, higher spending, dangerous levels of debt, and more burdensome regulations from Washington."

- When the disappointing jobs report came out on Friday, Portman blamed the President for an economy that has 5 million fewer jobs than before the recession, saying "Unfortunately, time and time again, the Obama administration chooses politics over policy and style over substance."

- And in an op-ed for Fox News on Saturday, Portman called the Obama administration "out of ideas and out of touch."

So does the tough new tone have anything to do with Portman's status as a potential Vice Presidential candidate?

Not at all, says Portman's spokesman.

"It reflects his increasing frustration with Washington's inability to fulfill its most basic responsibilities," says Portman spokesman Jeff Sadosky, although he argued that every time Portman criticizes the president, he also offers a serious alternative.